WASHINGTON — A mass shooting Monday morning at the Washington Navy Yard left at least 13 people dead, including a suspected gunman, who police late Monday night said was a contractor with valid military identification who apparently drove onto the base.

It was perhaps the most devastating shooting attack in the heavily guarded nations capital in decades and sparked new concerns about security at U.S. military facilities.

District of Columbia Police Chief Cathy Lanier identified the gunman as a civilian Navy contractor, Aaron Alexis, 34, of Fort Worth, Texas, who was identified by fingerprints. An employee of a Hewlett-Packard subcontractor, Alexis was a full-time Navy reservist until 2011, last serving with a logistics support squadron in Fort Worth.

At a late-night news conference, local police and the FBI dismissed earlier media reports that Alexis used a friends identification to get into a building at the Washington Navy Yard before he sprayed bullets on prone victims until police arrived, exchanging gunfire with him several times before he was shot dead. Alexis, they said, had a legitimate ID to enter the facility. Hewlett Packard released a statement Monday night confirming that Alexis worked for The Experts, a large subcontractor on a naval contract managed by HP Enterprise Services.

For much of the day, authorities were seeking to question a black man between 40 and 50 years of age described as wearing an olive-colored military-style uniform. Late Monday night, Lanier said Alexis was thought to have been a lone gunman. Lanier said earlier that a potential third suspect was cleared of any involvement.

The FBI also confirmed that a car left at the scene is believed to have been driven by Alexis.

We are processing a vehicle at the scene related to the shooting, said Valerie Parlave, the FBI assistant director-in-charge.

Late Monday CNN reported that the HP contract involved Alexis visiting offices all along the Eastern seaboard and that there had been no complaints about his work or behavior.

In its statement, HP said the contract Alexis was working on was to to refresh equipment used on the Navy Marine Corps Intranet (NMCI) network. HP is cooperating fully with law enforcement as requested.

Following the late-hour news conference, police released the first seven names of the slain vicitims. None were active-duty military. They were identified as Michael Arnold, 59; Sylvia Frasier, 53; Kathy Gaarde, 62; John Roger Johnson, 73; Frank Kohler, 50; Bernard Proctor, 46; and Vishnu Pandit, 61.The families of the remaining five victims had yet to be informed.

Alexis had a police record for gun-related incidents in the states of Texas and Washington, according to legal documents and law enforcement.

Multiple news reports said the Navy had discharged Alexis for a pattern of misconduct. His ability to get a clearance with a general discharge from the military and two gun arrests likely will become a key matter for investigation..

Nutpisit Suthamtewakul, owner of Happy Bowl Thai in the Fort Worth area, told the Fort Worth Star-Telegram that Alexis was my best friend.

He lived with me three years, Suthamtewakul said Monday afternoon. I dont think hed do this. He has a gun, but I dont think hes that stupid. He didnt seem aggressive to me.

The FBI posted a Seeking Information notice about Alexis, showing his photograph and asking for help in piecing together how and why he was in the nations capital.

We dont have any reason at this stage to suspect terrorism, said D.C. Mayor Vincent Gray, though he said it hasnt been ruled out.

A second-floor cafeteria off the atrium of the Naval Sea Systems Command headquarters, a workplace for 3,000 people, was bustling with morning traffic when one or more gunmen opened fire around 8:20 a.m. from a fourth-floor mezzanine, the Navy said. The wounded survivors described hearing a steady pop-pop-pop, but it was unclear whether the shooter had a semiautomatic weapon, said Dr. Janis Orlowski, the chief medical officer at the Medstar Washington Hospital Center.

Gary Humes, whose LinkedIn profile lists him as director of strategic operations for the Navy, told reporters that while inside the locked-down building, people searched their computers upon learning Alexis had been identified as the shooter. They found him listed as a computer specialist under contract in San Diego.

Navy contractor Sean Carroll described to McClatchy a chaotic scene once the shooting started.

People didnt realize what you were supposed to do, he said. Just heard the sounds. It was really loud. You could hear the gunshots. Thats a surreal thing. Youre not really thinking. But it wasnt like, Hey whats going on? You know with the world we live in. You know, Oh my God. This is Fort Hood.

Fort Hood in Texas was the scene in 2009 of a shooting by a member of the military that left 13 dead.

Police records in Fort Worth show that on Sept. 4, 2010, officers were dispatched to his apartment when a neighbor alleged he shot a hole up through his ceiling.

Alexis explained that the gun discharged after it slipped while he was cleaning it. He was arrested but the charges were never pursued.

Police in Seattle put out a statement saying that Alexis had been arrested on June 3, 2004, for an anger-fueled blackout in which he shot out the rear tires of a car at a construction site near his home. Construction workers said he had stared at them angrily for about a month before the shooting. Alexis, said the police statement, told them he didnt remember the shooting initially.

Detectives later spoke with Alexis father, who lived in New York at the time, who told police Alexis had anger management problems associated with post-traumatic stress disorder, and that Alexis had been an active participant in rescue attempts on September 11th, 2001, the Seattle police statement said.

The FBI, which took the lead in the investigation, was trying to learn more about the past of the alleged gunman, whom friends called a practicing Buddhist.

Our (intelligence) teams and our investigative teams are looking at those reports, and trying to come up with a better understanding of who he is and exactly what he was doing, FBI spokeswoman Lindsay Godwin said.

Security guards at the Navy Yard scrambled to engage the gunmen, and police officers arrived within two to three minutes, Lanier said, and shooter teams, assembled from several agencies, were on site within seven minutes. After several exchanges of gunfire, the assailant died in a final flurry with metro police, Lanier said.

Lanier credited D.C. police and the U.S. Park Police for preventing even more bloodshed in the morning rampage.

I think the actions by the police officers, without question, helped to reduce the numbers of lives lost, Lanier said, calling the actions of first responders nothing short of heroic.

This is a horrific tragedy, Gray said. I dont know if weve ever had one like this . . . nothing like this in the District of Columbia.

Three gunshot victims, including a police officer, were brought to MedStar Washington Hospital Center. In a press conference, Orlowski, the centers chief medical officer, said their chances of survival were good. She confirmed that all three victims were alert and talking when they arrived.

The wounded police officer was shot in both of his legs and underwent long hours of surgery to repair blood vessels and bones, she said. She said doctors would try to assess on Tuesday his chances of being able to walk again.

The other two wounded survivors are both female civilians. One suffered wounds to her shoulder and underwent hours of surgery. The other was shot in the head and hand, but luckily the bullet did not penetrate her skull and she didnt need surgery, Orlowski said.

President Barack Obama, who ordered flags at government buildings lowered to half-staff through Friday to honor the victims, vowed that his administration would do everything in our power to make sure whoever carried out this cowardly act is held responsible.

The neighborhood near the naval facility remained sealed off, with residents locked out and nearby schools locked down. The Senate was placed on a preventive lockdown early Monday afternoon, with votes postponed. The lockdown was later partially lifted. The Washington Nationals, whose stadium is just blocks from the crime scene, postponed their game Monday night against the Atlanta Braves.

The area around the Navy Yard, not far from Capitol Hill, had been sealed off by layers of law enforcement personnel from local and federal agencies. Senate Sergeant at Arms Terrance Gainer issued a statement midafternoon announcing that no one would be allowed in or out of Senate offices, but later lifted it partially.

Patricia Ward, a logistics management specialist, told reporters that she was near the cafeteria area when she heard some shots, followed by a break and then more shots. She and others fled the building. Someone pulled a fire alarm to alert colleagues of a problem.

The gunman was described by witnesses as carrying an AR-15 assault rifle, a double-barrel shotgun and a handgun.

An elite team from the Bureau of Alcohol, Tobacco, Firearms and Explosives was dispatched to the scene Monday, part of the same ATF Special Response Team Canine Program that helped capture the suspect in the Boston Marathon bombing.

Employees at the Navy Yard complex, a former shipyard and ordnance plant turned naval administrative center, consist of civilians, service members and contract support personnel. They are responsible for engineering, building, purchasing and maintaining Navy ships, submarines and combat systems. The Naval Sea Systems Command is the largest of the Navys five systems commands.

This is not the first shooting at a military installation in recent years. In 2009 at Fort Hood, an Army psychiatrist, Maj. Nidal Malik Hasan, 39 at the time, killed 13 and wounded 31 service members. Another, less well-known incident occurred on June 10, 2013, at Fort Sam Houston in San Antonio, where a soldier shot and wounded a fellow service member.

Theres something evil in our society that we as Americans have to work to eradicate, she said. I see this every day. Theres something wrong here when we have these multiple shootings. The only thing I can say is we have to work together to get rid of it. Id like you to put my trauma center out of business. Id like to not be an expert on gunshots.

White House Press Secretary Jay Carney said it was too early to talk about whether the situation would revive the presidents stalled gun control efforts.

This is an ongoing investigation; its an ongoing situation, Carney said. It would be inappropriate to try to put in context something about which we have so few facts.

In the firstlede writethru, it was incorrectly reported that Aaron Alexis worked or a Hewlett Packard subsidiary. The company The Experts is actually subcontractor for HP Enterprise Services. HP did not clarify the relationship further.

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